A Ziegler-Natta catalyst for the preparation of a polyolefin mainly composed of an organoaluminum compound and a transition metal compound of Groups III to V has heretofore been used with a compound having an electron donative group to enhance the activity thereof. The reason for the enhancement of the activity is unknown at present. The recent studies suggest that the electron donor to be used needs to satisfy the following requirements:
(1) The compound contains an ester group and an alkoxy group in its molecule; and
(2) The compound contains a bulky substituent which gives steric hindrance in its molecule.
The inventors previously confirmed on the basis of the foregoing presumption that the use of a 3-alkoxy-2-alkyl-substituted ester propionate represented by the general formula: EQU R.sup.1 --O--CH.sub.2 --CH(R.sup.3)COOR.sup.2
wherein R.sup.1, R.sup.2 and R.sup.3 each represent an alkyl group, can provide improved catalyst properties, as disclosed in JP-A-2-289604 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, many electron donors which have heretofore been proposed, including the foregoing electron donors, cannot necessarily satisfy high industrial requirements for catalyst properties such as high activity, high stereoregularity and high productivity any more.
The inventors presumed that one of the most effective solutions for improving the properties of a Ziegler-Natta catalyst is to use a compound having an ester group and an alkoxy group in its molecule and two bulky alkyl groups on the carbon atom at the .alpha.-position with respect to the ester group as an electron donor. None of these compounds have been reported in any references probably due to the steric hindrance of the bulky alkyl group at the .alpha.-position with respect to the ester group or probably because their application was not expected at all.
The inventors sought for a novel compound containing an ester group and an alkoxy group in its molecule and two alkyl groups, particularly bulky alkyl substituents on the carbon atom in a position alpha to the ester group on the basis of the foregoing presumption.
Heretofore, many solid catalyst components made of a magnesium compound, a titanium compound, a halogen compound and an electron donor as essential components have been proposed as catalyst components.
It is well known that these catalysts exhibit a high activity in the polymerization of olefin. It is also well known that these catalysts exhibit a high stereospecificity in the polymerization of .alpha.-olefin.
It is further known that such a solid catalyst component prepared from an alkyl phthalate compound as an electron donor can exhibit excellent properties on polymerization.
In a catalyst system comprising this ester phthalate compound as an electron donor, it is necessary that a large amount of an organosilicon compound be used as a co-catalyst component in addition to organoaluminum compound. However, the organosilicon compound to be used for the foregoing purpose has a bulky structure and thus is expensive. Thus, it causes a problem in the catalyst cost. Further, siloxanes or inorganic silicate produced by the decomposition of the foregoing organosilicon compound are microdispersed and left in the polymer and thus have a great affect on the physical properties of the polymer.
The inventors previously found that the use of an organic compound having a specific structure as an electron donor for catalyst can provide a solid catalyst component which can exhibit some stereoregularity without using an organosilicon compound as a co-catalyst component (as disclosed in EP38334A). However, practicable properties cannot be obtained only by the use of the solid catalyst and organoaluminum compound. Thus, in the actual use, such a solid catalyst component must comprise an organosilicon compound, though in a small amount, incorporated therein.
In recent years, studies have been made on the use of diether compounds as electron donors (as disclosed in JP-A-3-294302). However, diether compounds have a high potential that they have a physiological activity from the structural standpoint of view. Therefore, diether compounds must be carefully handled. Thus, diether compounds have problems in safety.
It is well known that an electron donor is used as a co-catalyst component of a magnesium chloride-supported Ziegler-Natta catalyst. Many reports have been made that alkyl benzoates, piperidines, acetals or organosilicon compounds (alkoxysilane) are useful as electron donors. However, among these compounds, alkyl benzoates or piperidines have an inherent odor. When these compounds are used as electron donors, an odor remaining in the product can often cause troubles. Further, acetal compounds are liable to deactivation of the catalyst, resulting in the increase of catalyst cost.
Accordingly, an organosilicon compound (alkoxysilane) is normally often used for the foregoing purpose. It is well known that an organosilicon compound, particularly alkoxysilane compound, can easily undergo hydrolysis due to water content in the air or the like, giving the corresponding silanol compound or siloxane compound. Therefore, if an organosilicon compound is used as an electron donor for catalyst, the resulting polymer often contains a silanol compound or siloxane compound.
Heretofore, such a catalyst residue, etc. has been removed by deliming process. In recent years, however, more olefin polymerization plants have employed a non-deliming process or non-pelletizing process more often. Therefore, it has happened more often that product polymers having catalyst residues left therein are on the market.
However, the silanol compound or siloxane compound microdispersed in the polymer can often cause the drop in the physical properties, particularly rigidity, of the polymer. Thus, all of the inherent physical properties of the polymer cannot be necessarily exerted.
It has been keenly desired to develop a catalyst system which is free of organosilicon compound as a co-catalyst component and causes no safety problems, and a catalyst system which exhibits excellent properties without using any organosilicon compound which is a source of a silanol compound or siloxane compound as an electron donative component for olefin polymerization catalyst.